Remove implicitly nullable parameter declarations

[MarkusBauer]

Implicitly nullable parameter declarations are deprecated from PHP 8.4 on and will be removed in PHP 9. This PR removes all nullable declarations to keep the code compatible with future PHP versions, with no impact on backward compatibility.

As an alternative to dropping declarations, we could use declarations of the form (?Matrix $ctm = null), but then we would lose compatibility with PHP before 7.1.

How to reproduce the issues:

> docker run --rm -v .:/app "php:8.4-rc-cli" find /app -type f -name "*.php" -exec php -d error_reporting=32765 -l {} \; | grep -v 'No syntax errors detected'
Deprecated: setasign\Fpdi\GraphicsState::__construct(): Implicitly marking parameter $ctm as nullable is deprecated, the explicit nullable type must be used instead in /app/src/GraphicsState.php on line 29
Deprecated: setasign\Fpdi\PdfParser\Type\PdfStream::parse(): Implicitly marking parameter $parser as nullable is deprecated, the explicit nullable type must be used instead in /app/src/PdfParser/Type/PdfStream.php on line 38
Deprecated: setasign\Fpdi\PdfParser\Type\PdfDictionary::get(): Implicitly marking parameter $default as nullable is deprecated, the explicit nullable type must be used instead in /app/src/PdfParser/Type/PdfDictionary.php on line 110

With the patches from this PR, the deprecation warnings are gone.

[JanSlabon]

Markus, thank you for the PR. What do you think about my comments?

[andrewnicols]

Is support for PHP 7.1 (and 7.2, 7.3) really something that is desirable, let alone PHP 5?

[MarkusBauer]

Is support for PHP 7.1 (and 7.2, 7.3) really something that is desirable, let alone PHP 5?

Not sure if desirable, but I wouldn't drop it for such a minimal change - in particular because it solves a problem we don't have yet. There are some really old PHP versions around in the LTS world. For example, RedHat's PHP 5.4 just went EOL a few weeks ago.

[JanSlabon]

We agree with @MarkusBauer and do not drop legacy support for such a minimal change for now.